(i) Field of the Invention
This invention relates to a hydraulic servo for friction coupling elements in automatic transmissions.
(ii) Description of the Prior Art
Heretofore, the hydraulic servo for a friction coupling elements in automatic transmission is constituted by an annular drum having a first cylinder which is an outer cylinder, a second cylinder which is an inner cylinder and a side wall connecting the two cylinders and fixedly or rotatably mounted in the transmission case, a piston provided between the first and second cylinders of the drum, and a return spring located outward of the first cylinder or on a wall portion of the piston, coupling and releasing the friction element by supplying and draining line pressure to and from a chamber between the drum and piston.
In a case where the return spring is located outward of the first cylinder, however, the hydraulic servo of the friction coupling element unavoidably has large dimensions in the radial direction. On the other hand, the arrangement in which the return spring is mounted on a wall portion of the piston has a drawback that the hydraulic servo of the friction coupling element becomes lengthy in the axial direction. Further, when upshifting a vehicular automatic transmission from first to second speed, the outer race of a one-way clutch F1 is fixed to the automatic transmission case by engaging a friction brake B2. In this connection, the hydraulic servo B-2 which engages or releases this brake B2 is constituted by an annular hydraulic drum and a piston which is biased by a return spring, usually involving a wide piston mounting space between the first and second cylinders of the hydraulic drum. Therefore, there is little space for mounting the return spring unless the case is extended to a certain extent to secure the return spring mounting space. In addition, shift shocks occur on upshifts from first to second speed due to the use of a large piston.